where \(I_1 = \lambda _1^2+\lambda _2^2-2\)  and \(I_2 = \lambda _1^2\lambda _2^2-1\)  are the strain invariants for the \(j\) -th element, while \( \lambda _1\)  and \( \lambda _2\)  are the principal stretch ratios [1]}, [2]}. Eq. (REF ) is made of two terms: the first one describes shear deformations, and the related elastic modulus is \(k_{\mbox{\scriptsize S}}\) ; the second one is introduced to describe area dilation, and the corresponding elastic modulus is \(k_{\scriptsize \alpha }\) . Note that the surface elastic shear modulus \(k_{\mbox{\scriptsize S}}\)  enters in the Capillary number \(\text{Ca}\)  defined as:
\(\text{Ca}= \frac{\mu _{\mbox{\scriptsize out}}\dot{\gamma }r}{k_{\mbox{\scriptsize S}}}\; .\) 
